Forms stores all the business processes and instance information in the Forms Database. There are no XML files stored anywhere by default, unless you have specifically exported your processes somewhere manually (Feature Request: how come you still can't select more than one process to export at a time, by the way?).

If you uninstall Forms, but the database is still intact somewhere, then you can always reinstall the same version of Forms and point it to the old database to recover your processes. It is a great idea to backup this database for this very reason.